The Stability of Pyridine and Its Derivatives upon Chemical Attack under the Conditions of Primitive Earth

Abstract:

Pyridine is a heterocyclic aromatic molecule of gross formula C₅H₅N with a six-membered ring similar to the structure of benzene where one CH group of the ring is replaced by a nitrogen atom. The molecule itself is not abundant in nature, but its derivatives are often part of important biomolecules. For instance, it is one of the basic units in the nicotinamide adenine dinucleotide, NADH, which is an essential reducing agent in biological processes. Pyridine derivatives (e.g. pyridine carboxylic acids) were identified in carbonaceous chondrites, while nicotinonitrile (3-cyanopyridine) as well as 2- and 4-cyanopyridine have been synthesized by the action of electric discharges on ethylene and ammonia, an intermediate step being the synthesis of pyridine, in Miller's experiments. The possibility that nicotinonitrile hydrolyzed in the primitive ocean to nicotinamide and nicotinic acid implies an important role of pyridine in prebiotic chemistry.

Either formed locally on Earth from simple precursors or brought by extraterrestrial carriers, the presence of pyridine or of one of its derivatives could have played an important role in the organic chemistry that triggered the origin of life on Earth. For this reason, in our laboratory we have undertaken a systematic experimental investigation to address pyridine stability in the conditions of primitive Earth. In a first series of experiments, we have exposed isolated pyridine molecules to the attack of very reactive species, namely atomic oxygen and nitrogen. The aim is to verify whether the N-containing aromatic ring of pyridine is preserved after the chemical attack of reactive transient species like O and N atoms that might have been relatively abundant under the conditions of primitive Earth when the O2/O3 UV shield was not yet present. The experimental technique employed is the crossed molecular beam technique with mass spectrometric detection. The implications for the stability of pyridine and its derivatives, or of other molecules for which pyridine can be considered a proxy, will be noted.

The Authors wish to thank the Italian Space Agency for co-funding the Life in Space project (ASI N. 2019-3-U.0).